1. Field of the Invention
The present invention relates to an active matrix substrate, a method manufacturing an active matrix substrate, and an intermediate transfer substrate for manufacturing an active matrix substrate.
2. Description of the Related Art
An active matrix type display device having active elements arranged on matrix-like pixels permits realizing a planar type display device of a high image quality. Particularly, a liquid crystal display device (LCD), in which a liquid crystal is used as an optical shutter and each pixel is driven by an active element such as a TFT, is widely used in various devices such as a PC monitor and a television receiver for a video display.
Also, an organic EL display device that permits displaying a full color image on a thin panel has been developed. In the organic EL display device, the organic EL materials that emit light rays of red, green and blue are formed into pixels by an ink jet method or a mask vapor deposition method, and each pixel thus formed is driven by an active element such as a thin film transistor (TFT).
In almost all the types of the display device, the active element is formed on a glass substrate. However, the glass substrate tends to be cracked and is heavy. Also, the display device having a glass substrate incorporated therein tends to be broken and is heavy. Such being the situation, it is desirable to develop a tough and lightweight display device. It is also desirable to develop a flexible display device that can be bent or folded freely.
Under the circumstances, a display device comprising a flexible substrate excellent in the impact resistance and light in weight such as a plastic substrate attracts attentions as a display device satisfying the requirements given above. In the display device of the particular type, it is necessary for an active element such as a thin film transistor (TFT) to be formed on the plastic substrate. Presently, amorphous silicon or polycrystalline silicon (polysilicon) is widely used for forming the thin film transistor. What should be noted is that it is absolutely necessary to employ a high temperature process of about 350° C. to 600° C. for forming the thin film transistor. On the other hand, the plastic substrate is resistant to heat of only up to about 200° C. It follows that it is difficult to form the thin film transistor directly on the plastic substrate.
As a method for overcoming the difficulty described above, proposed is a method of using an element formation substrate and a final substrate in place of the method of forming an active element directly on the plastic substrate. To be more specific, the element formation substrate is formed of a glass substrate having thin film transistors formed thereon at a high density so as to form a thin film transistor array. On the other hand, pluralities of plastic substrates are used as the final substrates. Of course, the thin film transistor array is transferred from the element formation substrate onto the plastic substrates used as the final substrates. The particular method is disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 6-118441, Japanese Patent Disclosure No. 11-142878 and Japanese Patent Disclosure No. 2001-7340. In this method, the thin film transistor equivalent in characteristics to the conventional thin film transistor can be formed on the plastic substrate because the thin film transistor can be formed at the temperature substantially equal to that for forming the conventional thin film transistor. It should also be noted that the transfer cost can be lowered because the thin film transistor array can be transferred from a single element formation substrate onto a plurality of final substrates.
In the conventional method, however, it is possible for even the element that should not be transferred to be transferred from the element formation substrate onto the final substrates so as to give rise to the problem that the transfer selectivity is lowered.